Ventilation Duct Diameter Calculator

Estimate round duct size

Enter required airflow in cubic feet per minute and a target air velocity in feet per minute. The calculator converts required cross-sectional area into the equivalent inside diameter of a round duct.

Five hundred CFM at 800 feet per minute requires about 0.625 square feet of area, equivalent to roughly a 10.7-inch round duct before selecting a standard size.

Velocity affects noise and resistance

A smaller duct increases velocity, noise, and friction loss. A larger duct lowers velocity but needs more installation space. Bends, reducers, filters, silencers, and flexible duct add resistance, so diameter alone cannot confirm fan performance.

Use a complete fan calculation

Check the fan curve at estimated static pressure, not only its free-air CFM rating. Long or complicated runs may require a larger duct or a different fan. Keep flexible duct straight and supported where it must be used.

Safety and installation

• Use compatible duct and fittings.
• Support equipment and seal joints correctly.
• Maintain required clearances.
• Follow local building, fire, and electrical rules.

This is an airflow geometry estimate. Final HVAC design should include pressure-loss calculations and qualified review.